Pesticides are considered as one of the main factors involved in environmental contamination of today's world. These chemicals are on purpose designed to be toxic to pest and vectors of diseases. These compounds are among more than 1000 active ingredients that are marketed as insecticide, herbicide, and fungicide. Nevertheless, formulation of new and potent pesticides is increasingly on the order of researchers and manufacturers because of pest resistance, hygienic controls, and major human need for more food as the world population grows. The use of insecticides in agriculture in recent years has increased tremendously and overshadowed all other control measures as means of crop protection. Although pesticides have largely benefited the human life through enhancement of agricultural products and controlling infectious diseases, their extensive use, in turn, has offended human health from side to side of occupational or environmental exposures. Among the various problems associated with pesticide use is the possibility of its biological accumulation in animals and plants. . Long-term contact to pesticides can harm human life and can disturb the function of different organs in the body, including nervous, endocrine, immune, reproductive, renal, cardiovascular, and respiratory systems. In this regard, there is mounting evidence on the link of pesticide's exposure with the incidence of human chronic diseases. In order to reduce negative impacts, it is desirable that pesticides be degradable or at least quickly deactivated in the environment. Such loss of activity or toxicity of pesticides is due to both innate chemical properties of the compounds and environmental processes or conditions. In addition, the US EPA uses the National Research Council’s four step process for human health risk assessment: Hazard identification, Dose-response assessment, Exposure assessment and Risk characterization. Hence the present seminar is aimed to interact and discuss on the degree of accumulation of different types of pesticides in different tissues of flora and fauna, their biotransformation, biomagnifications, the possible impact on them on the physiology and metabolism of flora and fauna including humans, and mechanisms of biodegradation